Comparator circuits for radio locators



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Patented Apr. 15, 1947 UNITED STATES PATENT OFFICE COMPARATOR CIRCUITS FOR RADIO .LOCATORS Delaware Application May 30, 1944, Serial No. 538,028

'7 Claims.

My invention relates to radio locator apparatus. It relates particularly to comparator circuits for comparing the amplitudes of reected signals that are received during successive antenna switching periods in a system employing a directive antenna system that has its region of maximum field strength angularly displaced at the switching rate, and to systems utilizing such comparator circuits for directing an object in flight toward a target or other signal reflecting object.

The invention will be described specifically as applied to a locator system of the frequencymodulated type having left-right directive antennas positioned to have overlapping radiation patterns. The invention will also be described as applied for automatic left-right or rudder control of an airplane carrying the radio locator system.

An object of the invention is to provide an improved method of and means for obtaining in radio locator apparatus a signal that is representative of or proportional to the angular amount by which the locator antenna system fails to point directly toward a target or other reflecting object. Another object of the invention is to provide an improved system including a follow-up circuit for causing a directive antenna system automatically to be pointed towards a target or other reflecting object.

Another object of the invention is to provide an improved comparator and follow-up circuit `for a radio locator system wherein directive antermas are switched successively to the locator system for obtaining directional information.

Still another object of the invention is to provide an improved comparator and follow-up circuit for an aircraft control system in which reflected radio signals automatically ccntrolthe direction of travel of the aircraft.

Still another object of the invention is to provide an improved circuit for comparing the amplitudes of reflected signals and in which there is a follow-up connection that acts without the introduction of time delay by an electrical iilter.

In a preferred embodiment of the invention two directive antennas with overlapping radiation patterns are switched alternately to the locator system, and a comparator or amplitude comparison circuit is switched synchronously with the antenna switching. The comparator circuit comprises a pair of rectiers in the audio frequency channel of the receiver which are switched alternately to supply signal to a filter (Cl. Z50-1.54)

that is balanced with respect to ground. A follow-up tap is provided on thel output resistor of the filter.

The single gure of the drawing is a block and circuit diagram of a radio locator system embodying the invention.

'I'he drawing shows the invention applied to a frequency-modulated radio locator system comprising a radio transmitter IU which is cyclically frequency modulated by a suitable frequency modulator II, such as a variable capacitor unit, that has a modulating signal I2 applied thereto from a wave shaping circuit I3. In the example illustrated, the modulating signal I2 is of triangular wave shape but it may be of some other wave form such as sinusoidal or sawtooth.

The triangular wave I2 may be produced by opening and closing a switch I4 in series with a battery I6 and a potentiometer resistor I1. This generates a square wave voltage I8 that is appliedto the wave shaping circuit I3 to obtain the triangular voltage I2. In this case the shaping circuit I3 may be merely an integrating circuit. The switch I4 is operated by a motor driven cam I 9. If desired, the wave shaping circuit and modulator described in application Serial No. 512,153, filed November 29, 1943, led in the name of Irving Woli and entitled Vibratory mechanical systems, may be employed.

The output of the transmitter I0 is radiated alternately from a pair of right-left directive `antennas T1 and T2. This is Vaccomplished by means of a suitable switch which, in the illustration, comprises switch arms 2I and 22 connected together mechanically by a block 23 of insulating material. The switch arms 2| and 22 are actuated by a cam 24 driven by a motor 26, the complete left-right switching cycle being to second, for example. Instead of switching the transmitter antennas, a pair of directive receiving antennas' may be switched, if preferred. Also, other known means of shifting the antenna system directivity may be employed in place of the means described.

Preferably, the frequency modulation sweep and the antenna switching are synchronized by driving the switch cam I9 from the same motor 26 that operates the antenna switchZI, 22. At the beginning the upsweep of each triangular wave I2 one of the transmitter antennas is connected to the transmitter II). In the example illustrated, there is one complete cycle of frequency modulation during each antenna switching period.

` After reection from a target, the frequency diode 3B back to theV coupling capacitor 35. 4the positive half cycle, the capacitor 3B is dismodulated signals are received by a directive antenna R and supplied to a beat frequency detector 21. The frequency modulated signals are also supplied to the detector 21 over a line 28 directly from the transmitter il) whereby the beat frequency of the detector output is proportional to the distance to the target as is well known in the art. This distance may be indicated by a frequency counter 29 supplied with signal through an amplitude limiter 25.

For obtaining directional information, output signal from the detector 21 is also supplied through an audio frequency amplifier 3| and through a coupling capacitor 30 to the anode 34 and cathode 36 of a pair of rectiliers such as diodes 31 and 38, respectively.

The received signals contain directional information because the amplitudes of the signals received during radiation from the right and left antennas T1 and T2, respectively, will not be zequal unless the antenna system comprising the pair of antennas T1 and T2 is pointing directly toward the reflecting target so that the target is in the region of equal iield strength of the two antennas.

An automatic gain control circuit 40 preferably -is provided for the audio ampliiier 3 I. If an automatic gain` control circuit is not provided and the signal strength becomes so large that limiting occurs in the audio ampliiier, then the output of the audio amplifier will be a signal that looks as lthough the antenna is pointed toward the target regardless of its direction with respect to the 1 target.

Referring more specifically to the left-right control portion of the system, the diodes 31 and r38 have an output resistor 4| connected between their cathode 42 and anode 43. The electrodes 42' and 43 are connected through conductors 44 and 46, respectively, to the contact points of a switch 45 having a switch arm 41 which is connected to ground. In somecases it may be pre- -ferred to connect the switch arm 41 to a positive potential point on a low impedance voltage divider resistor (not shown) for providing some of the bias on the amplifier tube 64 referred to hereinafter.

The electrodes 42 and 43 are connected alternately to ground by means of the switch 45.

Thus, either of the diodes 31 or 38 is operative to supply current through the output resistor 4| sistor 4| during the positive half cycle. During the negative half cycle the diode 38 is conducting whereby the coupling capacitor 30 is discharged substantially to ground potential. Similarly, when the switch arm 41 is in the right-hand position, Acurrent flows during the negative half cycle from ground through switch ann 41, the conductor 44 and through the load resistor 4| and the On charged substantially to ground potential through lthe diode 31 whereby blocking of capacitor 30 Adue to a direct-current charge is prevented.

A lter .52 that is balancedwith respect to ground is connected across the resistor 4|. The

-iilter 52 comprises series resistors 5B and 51 and shunt capacitors 58 and 59 which are connected -from the iilter resistors 56 and 51, respectively,

to ground. A lter output resistor 6| is connected across the capacitors 58 and 59. The resistive and capacitive values of the filter 52 are such as to smooth out current varying at the switching rate. A variable tap 62 on the output resistor 6| which functions as a signal output tap and as a follow-up tap, is connected electrically to the control grid 63 of an amplifier tube 64. If follow-up operation is desired, the tap 62 is connected mechanically, as indicated by the broken line 65, to a reversible motor 66 which is controlled by the output of the amplifier tube 613. The cathode 61 of amplifier tube 6ft is connected to a point on a voltage divider GB which holds the tube 64 properly biased for arnpliiier operation.

The direct-current output of the amplifier tube 64 iiows through the relay coils 1|a and 12a of a high current marginal relay 1| and a low current marginal relay 12 having switch arms 1 b and 12b, respectively. These switch arms are connected to make the reversible motor 56 run in one direction or the other, depending upon the relay energization as explained below whereby the airplane is turned in one direction or the other by means of a mechanical connection, indicated at 13, tothe airplane rudder or left-right control system. In the specific example shown, the D.C. output of tube 64 so adjusted by adjusting the grid bias that when there is no received signal, and also when the antennas T1 and T2 point directly toward the target, only the relay 12 is energized and the relay switch arms 1lb and 12b are in the positions illustrated. If the D.C. output of tube B4 increases above this neutral direct-current value, the relay 1| is also energized and the motor 56 runs in one direction to turn the airplane in one direction; if, on the other hand, the D.C. output decreases below this neutral" value, both relays are deenergized and the rnctor E6 runs in the other direction to turn the airplane toward the opposite direction. In one specific design the relay 12 opened on 3.4 milliamperes while the relay 1| opened on 4.3 milliamperes. The relays 12 and 1| closed at 4 milliamperes and 6 milliamperes, respectively.

Considering the circuit operation more in detail, when the right antenna T1 is connected to the transmitter I8, rectified current flows from diode 31 through resistor 4| to ground by way of the switch arm 41. At this time the iilter capacitor 53 is connected across resistor 4| through filter resistor 56 and through ground. During the next switching period, the antenna T2 is connected to the transmitter I3, and rectified current fiows from groundv through the switch arm 4.1, the lead 44, the resistor 4| and through the diode 38. This time the iilter capacitor 59 is connected across resistor 4| through filter resistor 51 and through ground. It is evident that the direction of charge for capacitors 58 and 59 is such that their terminals remote from ground are positive and negative, respectively. If the amplitudes of the received signals during the two switching periods are equal, the filter capacitors 53 and 59 are charged equally and the midpoint of resistor 6| is at ground potential. With the tap 52 at said midpoint, the output of amplifier Se has a value such that the relay 1| is deener-v gized, the relay 12 is energized, and the motor motor 66 is thus disconnected from a battery or Vother power supply 16.

If the pair of antennas Ti and T2 are directed to one side of the target whereby it is not in the region of equal field strength of the two antennas, thei-i the reflected signals during the two `switching periods will be unequal and the filter capacitors 58 and 59 will have unequal charges. Therefore, the tap 62 (assuming it is at the midpoint of resistor 6|) will not be at ground potential and either both of the relays H and 12 will be energized or both will be deenergized, depending upon whether the left signal or the right signal is the greater, whereby the battery 1B will be connected with the proper polarity to the armature 11 of the motor 56. Therefore, the motor 66 rotates in the direction to turn the airplane and the antennas Ti and T2 toward the target by means of the connection 13 to the airplane controls. When the antennas Ti and T2 point directly toward the target 'so that it is in the region of equal field strength of the two antennas, the midpoint of resistor 6l again is at ground potential and the motor BS stops.

When it is desirable to employ a follow-up control to prevent the airplane (and the antennas) `from being turned rst too far to the right and then too far to the left, the mechanical connection 65 may be provided between the adjustable tap 62 andthe motor B6. Then, as the airplane is turned toward one direction, the tap 62 is moved along resistor El toward the electrical ground point of the resistor 6 l.

It will be evident that the motor 55 will reverse and begin moving the rudder back to its center or neutral position before the airplane has been turned enough to point the antennas Ti and T2 directly toward the reecting target. By the time the antennas Ti and T2 are pointing directly toward the target, the rudder is back to its neutral position.

The amount that the motor Gt moves from its center position, and hence the rate of turn of the airplane, is a function of the ratio of right to left signal and hence the number of degrees oir course of the airplane.

It will be apparent that if there is more right signal through diode 31 than left signal through diode 38 whereby capacitor 58 receives the larger i charge, the amount of control by the right signal will be reduced as desired by the followup action while the airplane is turning toward the desired direction if the tap S2 is being moved toward the upper end of resistor BI during this time, since the ground point on resistor 6l has shifted toward the upper end of the resistor.

Thus by means of the balancing bridge action of the motor 66 and the resistor l the airplanes rate of turn is always proportional to the degrecs oil? course of the airplane with respect to the target. Ir the follow-up ratio of rate of turn of the airplane to the degrees off course is made small enough, hunting of the airplane will be prevented.

One of the advantages of the present invention is that the follow-up tap 62 is at the direct-current end of the rectifier-filter combination so that the filter 52 will not introduce any time lag in the transfer of a voltage change from the follow-up tap 62 to the grid S3 of the amplifier tube 6d. In systems where the follow-up tap precedes the filter, the lag may be great enough to adversely affect the operation 'of the followup circuit and cause hunting of the motor unless suitable precautions are taken.

If it is desired to apply the present invention to an aircraft that is to be flown without a pilot, i. e., to apply it to a drone, the aircraft may be held at a predetermined altitude by means of a system such as described and claimed in the copending application Serial No. 484,458, filed April 24, 1943, in the names of Royden C. Sanders, Jr., and John H. Purl,` and entitled Aircraft control system.

In the drawing various voltages and circuit values have been given merely by way of example. The values are given in ohms, thousands of ohms and megohms and in microfarads, the letter M indicating where ohms are given in thousands.

I claim as my invention:

l. In a direction finding system, a directive antenna system and means for angularly displacing its region of maximum eld strength during successive switching periods, a radio receiver, a pair of rectiers each having an anode and a cathode, a couplingr capacitor, the anode of one rectier and the cathode of the other rectier being connected through said coupling capacitor to the output circuit of said receiver, an output impedance unit connected between the other anode and the other cathode of said rectierS, switching means for making one of said rectiers pass current through said output impedance unit during one antenna switching period and for making the other of said rectiiiers pass current in the same direction through said impedance unit during the next antenna switching period, a iilter connected across said impedance unit, the midpoint of said filter being grounded, an output impedance unit connected across the output terminals of said lter, and an output tap on an intermediate point on said output impedance unit.

2. In combination, a radio locator system comprising a transmitter for transmitting a radio signal to a reflecting object and a receiver for receiving said signal after reiiection from said object, said system also comprising a directive antenna system and means for angularly displacingV its region of maximum field strength during successive switching periods, a pair of rectifiers each having an anode and a cathode, a coupling capacitor, the anode of one rectifier and the cathode of the other rectier being connected through said coupling capacitor to the output circuit of said receiver, an output impedance unit connected between the other anode and the other cathode of said rectifiers, switching means for making one of said rectifiers pass current through said output impedance unit during one antenna switching period and for making the other of said rectii'lers pass current in the same direction through said impedance unit during the next antenna switching period, a filter connected across said impedance unit, the midpoint of said filter being grounded, an output impedance unit connected across the output terminals of said filter, and an output tap on an intermediate point on said output impedance unit.

3. In a direction iinding system, a directive antenna system and means for angularly displacing its region of maximum field strength during successive vswitching periods, a radio receiver, a pair of rectiers each having an anode and a cathode, a coupling capacitor, the anode of one rectier and the cathode of the other rectifier being'eonnected through said coupling capacitor to the output circuit of said receiver, an output impedance unit connected between the other anode land the other cathode of said rectifiers, switching means for connecting one end of said output impedance unit through a low impedance connection to ground during one antenna switching period and for connecting the other end of said output impedance unit through a low impedance connection to ground during the next antenna switching period, a filter connected across said impedance unit, the midpoint of said lter being grounded, an output impedance unit connected across the output terminals of said lter, and an output tap on an intermediate point on lsaid lter output impedance unit.

4. In combination, a radio locator system comprising a transmitter for transmitting a radio signal `to a reecting object and a receiver for receiving said signal after reflection from said object, said system also comprising a directive antenna system and means for angularly displacing its region of maximum eld strength during successive switching periods, a pair of rectiiiers each having an anode and a cathode, a coupling capacitor, the anode of one rectier and the cathode of the other rectifier being connected through said coupling capacitor to the output circuit of said receiver, an output impedance unit connected between the other anode and the other cathode of said rectiiiers, switching means for connecting one end of said output impedance unit through a ow impedance connection to ground during one antenna switching period and for connecting the other end of said output impedance unit through a low impedance connection to ground during the next antenna switching period, a iilter connected across said impedance unit, the midpoint of said filter being grounded, an output impedance unit connected across the output terminals of said filter, and an output tap on an intermediate point on said lter output impedance unit.

5. In combination, a radio locator system comprising a tnansmitter for transmitting a radio signal to a reflecting object and a receiver for receiving said signal after reection from said object, said system also comprising a directive antenna system and means for angularly displacing its region of maximum field strength during successive switching periods, a pair of recters each having an anode and a cathode, a coupling capacitor, the anode of 'one rectifier and the cathode of the other rectier being connected through said coupling capacitor to the output circuit of said receiver, an output impedance unit connected between the other anode and the other cathode of said rectiliers, switching means for making one of-said rectifiers pass current through said output impedance unit during one antenna switching period and for making the other of said rectiers pass current in the same direction through said impedance unit during the next antenna switching period, a filter connected across said impedance unit, said lter including shunt capacitors connected between each side of said iilter and ground, a resistor Connected across the output terminals of said lter, and a tap on said resistor for supplying an output signal.

6. In combination, a radio locator system comprising a transmitter for transmitting a radio signal to a reecting object and a receiver for receiving said signal after reflection from said object, said system also comprising a directive antenna system and means for angularly displacing its region of maximum :lield strength during successive switching periods, a pair of rectiers each having an anode and a cathode, the anode of one rectifier and the cathode of the other rectier being connected to the output circuit of said receiver, an output impedance unit connected between the other anode and the other cathode of said rectiers, switching means for making one of said rectiers pass current through said output impedance unit during one antenna switching period and for making the other of said rectiiers pass current in the same direction through said impedance unit during the next antenna switching period, a filter connected across said impedance unit to obtain a direct-current voltage, said filter including shunt capacitors connected between each side of said lter and ground, a resistor connected across the output terminals of said lter, a Variable tap on said resistor, an amplier having an input electrode connected to said variable tap, and a servo means including said amplifier for turning said antenna system toward said reflecting object in response to said region being more to one side of said object during one of said switching periods than to the other side of said object during the other switching period.

7. In combination, a radio locator system comprising a transmitter for transmitting a radio signal to a reflecting object and a receiver for receiving said signal after reflection from said object, said system also comprising a directive antenna system and means for angularly displacing its region of maximum field strength during successive switching periods, a pair of rectilers connected to rectify the output of said receiver, an output impedance unit for said rectiers, switching means for making one of said rectiers pass current through said impedance unit during one antenna switching period and for making the other of said rectiers pass current in the same direction through said impedance unit during the next antenna switching period, a lter connected across said impedance unit to obtain a directcurrent voltage, said filter including shunt capacitors connected between each side of said filter and ground, a reversible motor, relay means for controlling the direction and amount of rotation of said motor in accordance with the direct-current output of said lilter, means for controlling the direction in which said region of maximum iield strength is pointed, and a servo connection between said motor and said last means for turning said antenna system toward said reflecting object in response to said region being more to one side of said object during one of said switching periods than to the other side of said object during the other switching period.

WILLIAM R. MERCER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Name Date Moueix Oct. 17, 1939 Number 

